1. Field of the Invention
The present invention relates to a surface acoustic wave device used with portable telephones, carphones, radio equipment, etc., and a fabrication process thereof.
2. Description of the Background
Surface acoustic wave devices, i.e., surface acoustic wave filters and surface acoustic wave resonators are now increasingly used as substitute filters for dielectric filters in the high-frequency parts of portable telephones, etc. Some reasons for this are that the surface acoustic wave filters are smaller in device size than, and superior in electrical properties on the same device size basis, to dielectric filters.
A surface acoustic wave device comprises a piezoelectric substrate formed of lithium niobate, lithium tantalate or the like and various electrodes such as interdigital transducers (IDTs) and reflectors formed on the surface of the substrate. For the electrodes of the surface acoustic wave device, Al or Al alloys (e.g., Alxe2x80x94Cu) are ordinarily used. However, Al and Al alloys are susceptible to corrosion in a high-humidity environment, which may otherwise cause breaks in the electrodes or yield corrosion products, resulting possibly in a deterioration in filter performance. In a method used so far to prevent the deterioration in filter performance due to electrode corrosion, the electrodes are hermetically sealed up in a ceramic package to shield them against the environment.
With the method for prevention of electrode corrosion by hermetically sealing up the electrodes in the ceramic package, however, minute hermeticity breaks are likely to occur. For this reason, an additional step of inspecting the degree of hermeticity must be provided after the package is sealed. This inspection step is one factor of fabrication cost increases. In view of fabrication cost reductions, a resin package is preferable to the costly ceramic package. However, the resin package is inferior in humidity resistance to the ceramic package.
A method for prevention of IDT corrosion by covering the surface of the piezoelectric substrate having an IDT formed thereon using a thin film is also known in the art.
For instance, JP-B 3-190311 discloses a surface acoustic wave device comprising a piezoelectric substrate and a hydrophobic and insulating film of 100 xc3x85 or less in thickness, which is formed on the surface of the substrate having electrodes provided thereon. In the publication hexamethyl-disilazane, azide compounds and isocyanate compounds are referred to for the material to constitute the aforesaid film, and in one example a hexamethyl-disilazane film of 50 xc3x85 or less in thickness is formed by spray coating. However, the publication reveals that there is an insertion loss increase of 0.2 to 0.3 dB as a result of the formation of this film.
JP-A 4-294625 discloses a surface acoustic wave device wherein, in order to prevent a release and corrosion of electrode material, a protective film comprising a corrosion-resistant metal such as chromium or a dielectric material such as silicon dioxide is formed by sputtering or deposition-by-evaporation on the surface of a piezoelectric substrate having electrodes provided thereon. In one example in the publication, a chromium protective film of 100 xc3x85 in thickness is formed.
Some surface acoustic wave devices, wherein a thin film is formed on the surface of a piezoelectric substrate having electrodes provided thereon if not for the purpose of preventing electrode corrosion, are also known from the following publications.
JP-B 2-47886 discloses a process for resonant frequency control by the deposition-by-evaporation of a metal on the whole or a part of the surface of a piezoelectric substrate. In the publication, Ag, Au, Cr, Ni, etc. are referred to as the metal to be deposited by evaporation. The publication also teaches that there is no possibility of short circuits between electrode fingers because the amount of the metal to be deposited by evaporation is so small that it can be deposited in the form of discrete, fine points.
U.S. Pat. No. 3,965,444 discloses that for the purpose of compensation for temperature, an SiO2 film is formed on the surface of a piezoelectric substrate having electrodes provided thereon.
JP-A4-150512 discloses that a semi-insulating thin film is provided on the surface of a piezoelectric substrate having electrodes formed thereon. In the publication silicon oxide films, silicon oxide nitride films, silicon nitride films and silicon carbide are referred to as the material to constitute the semi-insulating thin film. By the provision of this semi-insulating thin film, static electricity generated at the piezoelectric substrate is discharged between electrodes in such a manner that any electrostatic breakdown of the device can be prevented. In one example in the publication, a silicon nitride film of about 500 xc3x85 or greater in thickness is formed as the semi-insulating film by means of a plasma CVD process.
JP-A 9-83288 discloses that in order to prevent discharge between electrodes, a semiconductor thin film is formed on the surface of a piezoelectric substrate having electrodes provided thereon, and in one example given therein an Si film of 50 nm in thickness is used as the semiconductor thin film.
JP-A 9-199974 discloses that in order to reduce noise generation due to the pyroelectric effect of a piezoelectric substrate, a resistor thin film is formed on the surface of the piezoelectric substrate having electrodes provided thereon. In the publication, a silicon thin film formed by sputtering or deposition-by-evaporation is referred to as the resistor thin film. However, the publication says nothing about the thickness of the resistor thin film.
JP-A 7-326942 discloses a surface acoustic wave device which includes an insulating film on the surface of a piezoelectric substrate having electrodes formed thereon and satisfies kh less than 0.15 provided that k=2xcfx80/xcex, where h is the thickness of the insulating film and xcex is the wavelength of a surface acoustic wave. The publication shows that the insulating film is provided for center frequency control. The publication also shows that the lower limit to kh is 0.002, and the amount of the then frequency change is about 80 ppm. In the examples in the publication, an MgO film and an SiO2 film formed by sputtering are used as the insulating film.
As explained above, it is known to provide the surface of the piezoelectric substrate having electrodes formed thereon with various thin films. As shown in the aforesaid JP-B 3-190311, however, the provision of the organic film of about 100 xc3x85 in thickness causes insertion losses to become worse. As described in the aforesaid JP-A 4-294625, the formation of the Cr film of 100 xc3x85 in thickness causes short circuits to occur between electrodes, resulting in a lowering of electrical properties.
When the thin film provided on the surface of the substrate having electrodes formed thereon is a dielectric film, too, some performance deterioration, if not large as in the case of a metal film, occurs due to short circuits. For instance, as can be noted from the aforesaid JP-A 7-326942 where center frequency is controlled by the provision of the dielectric film, even the insulating film has still some influences on device performance.
In view of such situations as mentioned above, an object of the present invention is to provide a surface acoustic wave device comprising electrodes formed of Al or an Al alloy, the humidity resistance of which is improved without having any adverse influence on its electrical properties.
The aforesaid object is achievable by the following embodiments of the invention
(1) A surface acoustic wave device comprising a metal oxide layer formed on a surface of a piezoelectric substrate having electrodes provided thereon, which electrodes comprise Al or an Al alloy, in such a way as to cover at least said electrodes, wherein said metal oxide layer is formed by oxidization of a metal layer having a thickness small enough to provide no continuous film.
(2) The surface acoustic wave device according to (1) above, wherein said metal oxide layer provides a continuous film at least on said electrodes.
(3) The surface acoustic wave device according to (1) or (2) above, wherein said metal oxide layer has a thickness of 0.1 to 2 nm as measured by a fluorescence X-ray thickness meter.
(4) A surface acoustic wave device comprising a metal oxide layer formed on a surface of a piezoelectric substrate having electrodes provided thereon, which electrodes comprise Al or an Al alloy, in such a way as to cover at least said electrodes, wherein said metal oxide layer has a thickness of 0.1 to 2 nm as measured by a fluorescence X-ray thickness meter.
(5) A surface acoustic wave device comprising a metal oxide layer formed on a surface of a piezoelectric substrate having electrodes provided thereon, which electrodes comprise Al or an Al alloy, in such a way as to cover at least a part of said surface of said piezoelectric substrate and said electrodes, wherein said metal oxide layer provides a continuous film on said electrodes and a discontinuous film on said surface of said piezoelectric substrate.
(6) The surface acoustic wave device according to any one of (1) to (5) above, wherein said metal oxide layer contains an oxide of a transition metal.
(7) The surface acoustic wave device according to (6) above, wherein said metal oxide layer contains a Cr oxide.
(8) A surface acoustic wave device fabrication process by forming a metal layer having a thickness small enough to provide no continuous film on a surface of a piezoelectric substrate having electrodes provided thereon, which electrodes comprise Al or an Al alloy, in such a way as to cover at least said electrodes, and then oxidizing said metal layer, thereby converting said metal layer to a metal oxide layer providing a continuous film at least on said electrodes.
(9) The surface acoustic wave device fabrication process according to (8) above, wherein said metal layer has a thickness of 0.1 to 2 nm as measured by a fluorescence X-ray thickness meter.